Efficient decomplexation of heavy metal-EDTA complexes by Co2+/peroxymonosulfate process: The critical role of replacement mechanism

A promising strategy to treat wastewater containing heavy metal complexes (HMCs) is to employ oxidation processes before carrying out the conventional processes. Metal ion-catalyzed oxidation processes (MICOPs) have been confirmed to be suitable for the remediation of wastewater. However, the treatment of HMCs by MICOPs has seldom attracted researchers’ attention and the key mechanisms remain unclear. In this study, Cu II -ethylenediamine tetraacetic acid (EDTA) was used to evaluate the decomplexation efficiency of various MICOPs, among which the Co II /peroxymonosulfate (PMS) system provided the best results. In-depth analysis of the decomplexation mechanism of Cu II -EDTA indicated that the high affinity of Cu(II) to EDTA inhibits the exchange between Co II and Cu II in Cu II -EDTA, thus preventing the chelation of the catalytically active Co 2+ with EDTA, which leads to the rapid decomplexation of Cu II -EDTA. Moreover, through thermodynamic computation, Mn II -EDTA, Cd II -EDTA, and Ni II -EDTA were selected as other HMC candidates to investigate their influence on the oxidation efficiency of the Co II /PMS system. The results confirmed that the oxidation efficiency of Co II /PMS is strongly dependent on the metal-ion replacement. Finally, we propose that the distinct performance of the Fenton(-like) process in decomplexing the Ni II -EDTA and Cu II -EDTA could be well explained by the metal-ion replacement mechanism.